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Veröffentlicht: Donnerstag, 19. August 2021 12:49

In a world where interaction of intelligent devices has become the driving force of industry, robust and swift communication is paramount. The same applies to information technology where large amounts of data must be transmitted reliably at high speeds. Achievable data rates are primarily determined by the properties of the transmission channel requiring sophisticated adaptation for best utilization.

This article highlights the principles and realization of a high-performance software defined COFDM-modem, designed to make the best use of existing 2-wire or 4-wire media such as power lines, sub-sea cables, twisted-pair wires. The modem can also be used in wireless applications.

Challenges

Transmission channels can be of poor quality, because of high attenuation, high noise levels or other impairments present in home, industrial or automotive environments. To best exploit a given channel, it needs to be characterized in terms of attenuation and noise spectral density to be optimized for optimal signaling and use of its capacity.

Depending on the determined channel properties, the modem must be configured in terms of positioning the transmission band, its bandwidth, and methods of modulation.

The optimal signal code construction (SCC) must produce the maximum effective modem data rate. In solving the optimization problem, two restrictions must be fulfilled: the performance of the modem signal must not exceed the maximum permissible power and the error rate must remain below a given upper limit.

Principles

The software-defined C-OFDM modem uses algorithms for immediate resolution of the optimization problem. The results of the optimization represent the structure of the SCC, which consists of the following parameters: bandwidth of the signal (Nyquist frequency), number of active (QAM-modulated) beams and distribution of QAM-alphabets. The modem uses an efficient combination of 4-dimensional Trellis coded modulation (4-D TCM) and the Trellis shaping according to Forney. Such SCC achieves a channel capacity just 3.5 dB short of the theoretical limit.

Adaptive methods within the modem include an adaptive front-end filter for optimal shortening of the pulse response of the channel and a two-stage adaptive equalizer (Kalman algorithm) in the frequency domain. All adaptive systems operate according to the decision feedback principle, with remodulation of the SCC being part of the maximum likelihood (ML) decoder to form a reference signal for adaptive systems. The erroneous decisions of the ML decoder are excluded from the decision feedback loop controlled by CRC error detection.

The high-precision synchronization of the modem, based on the linear phase regression of the two pilot tones and the corresponding Kalman filter, ensures accurate tracking of both clock and frequency offsets.

Solutions

The modem we present automatically adapts all settings during the initial handshake phase which typically completes within less than one second. The resulting performance is close to the theoretical limit yielding data rates of up to 11 bits/sec/Hz. This is more than twice the typical data rate of other commercially available OFDM modems. FDD (Frequency Division Duplex) and TDD (Time Division Duplex) modes are supported, both symmetric or asymmetric.

Furthermore, the modem can be used in almost any topology, point-to-point or complex point-to-multipoint where the analysis and optimization are performed automatically in a network of modems. Whenever channel properties change, the modem adapts to the new conditions automatically within a fraction of a second.

Applications

Applications include A/V intercoms, power grid communication, emergency light supervision, smart metering, in-vehicle communication and many more. In many cases, the modem can be used in existing bus topologies, e.g. replacing RS-485 networks or other simple infrastructure.

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